In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The background supporting such a rapid advance is a reduced wavelength of the light source for exposure. The change-over from i-line (365 nm) of a mercury lamp to shorter wavelength KrF laser (248 nm) enabled mass-scale production of dynamic random access memories (DRAM) with an integration degree of 64 MB (processing feature size≦0.25 μm). To establish the micropatterning technology necessary for the fabrication of DRAM with an integration degree of 256 MB and 1 GB or more, the lithography using ArF excimer laser (193 nm) is under active investigation. The ArF excimer laser lithography, combined with a high NA lens (NA≧0.9), is considered to comply with 65-nm node devices. For the fabrication of next 45-nm node devices, the F2 lithography of 157 nm wavelength became a candidate. However, because of many problems including a cost and a shortage of resist performance, the employment of F2 lithography was postponed. ArF immersion lithography was proposed as a substitute for the F2 lithography. Efforts have been made for the early introduction of ArF immersion lithography (see Proc. SPIE, Vol. 4690, xxix, 2002).
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water and ArF excimer laser is irradiated through the water. Since water has a refractive index of 1.44 at 193 nm, pattern formation is possible even using a lens with NA of 1.0 or greater. Theoretically, it is possible to increase the NA to 1.44. The resolution is improved by an increment of NA. A combination of a lens having NA of at least 1.2 with ultra-high resolution technology suggests a way to the 45-nm node (see Proc. SPIE, Vol. 5040, p 724, 2003).
Several problems associated with the presence of water on a resist film were pointed out. For example, profile changes occur because the acid once generated from a photoacid generator and the basic compound added to the resist can be partially dissolved in water. The pattern collapses due to swelling. It is also pointed out that water droplets remaining on the resist film, though in a minute volume, can penetrate into the resist film to generate defects. To overcome these drawbacks of the ArF immersion lithography, it was proposed to provide a protective coating between the resist film and water (see the 2nd Immersion Workshop, Resist and Cover Material Investigation for Immersion Lithography, 2003).
In the lithography history, the protective coating on the photoresist layer was studied as an antireflective coating. For example, the antireflective coating on resist (ARCOR) process is disclosed in JP-A 62-62520, JP-A 62-62521, and JP-A 60-38821. The ARCOR process involves forming a transparent antireflective coating on top of a photoresist film and stripping it after exposure. When the antireflective coating is made of perfluoroalkyl compounds (e.g., perfluoroalkyl polyethers or perfluoroalkyl amines) having a low refractive index, the light reflection at the resist/antireflective coating interface is minimized so that the dimensional precision is improved. In addition to these materials, the resist protective coating materials proposed thus far include amorphous polymers such as perfluoro(2,2-dimethyl-1,3-dioxol)-tetrafluoroethylene copolymers as disclosed in JP-A 5-74700. Since these fluorinated compounds are less compatible with organic substances, fluorocarbon solvents are used in coating and stripping of resist protective coating material, raising environmental and cost issues.
Other resist protective coating materials under investigation include water-soluble or alkali-soluble materials. See, for example, JP-A 6-273926, Japanese Patent No. 2,803,549, and J. Photopolymer Sci. and Technol., Vol. 18, No. 5, p 615, 2005.
The water-soluble protective coating, however, cannot be used in the immersion lithography because the overlay on that coating is water or a similar liquid. In contrast, since the alkali-soluble resist protective coating material is strippable with an alkaline developer, it eliminates a need for an extra stripping unit and offers a great cost saving. From this standpoint, great efforts have been devoted to develop water-insoluble, alkali-soluble resist protective coating materials. For instance, a resist protective coating material comprising a methacrylate resin having fluorinated alcohol on side chain has been proposed.
Required for the resist protective coating material are not only the ability to prevent the generated acid and basic compound in the photoresist film from being leached out in water, but also such properties as water repellency and water slip. Of these properties, water repellency is improved by introducing fluorine into the resin and water slip is improved by combining water repellent groups of different species to form a micro-domain structure, as reported, for example, in XXIV FATIPEC Congress Book, Vo. B, p 15 (1997) and Progress in Organic Coatings, 31, p 97 (1997).
However, introducing fluorine into resins for the purpose of improving sliding angle, receding contact angle or the like results in resins with a greater contact angle with the alkaline developer, which in turn results in increased development defects.
Of recent concern are defects, so called “blob defects,” occurring on the surface of a resist film after development. It is known that there is a tendency that these defects occur frequently in unexposed areas of a resist film and in a resist film having higher water repellency. As a general rule, a resist film having higher water repellency has a greater contact angle with water so that water remaining on the resist film surface assumes a high internal energy state during spin drying after development. The internal energy reaches maximum immediately before drying and causes damages to the resist film surface at the same time as evaporation of water, resulting in blob defects. In view of this mechanism, the contact angle on the resist surface after development must be reduced in order to prevent blob defects from occurring on the resist film.
When a resist protective coating having high water repellency is applied in order to improve sliding angle, receding contact angle and the like, the contact angle on the resist surface increases due to the intermixing between the resist film and the protective coating, allowing for a likelihood of blob defects. Use of a hydrophilic resist protective coating controls blob defects, but provides a smaller receding contact angle, which interferes with high-speed scanning and allows water droplets to remain after scanning, giving rise to defects known as water marks. There exists a demand for a resist protective coating which has a greater receding contact angle and a smaller contact angle on the resist surface after development.
The resist protective coating material is needed not only in the ArF immersion lithography, but also in the electron beam (EB) lithography. The resist undergoes changes in sensitivity during EB lithography for mask image writing or the like. The resist sensitivity changes due to evaporation of the acid generated during image writing, evaporation of vinyl ether produced by deprotection of acetal protective groups, or the like, as discussed in JP-A 2002-99090. It is desired to suppress resist sensitivity variation by coating a protective coating on top of the resist layer.